1. The Field of the Invention
This invention relates generally to measuring analytes in samples and, more specifically, to measuring analytes based on an electromagnetic spectrum that is characteristic of the analyte, for example as can be used to make non-invasive measurements of analytes in biological organisms.
2. Background and Relevant Art
Many attempts have been made to create appropriate apparatus for the non-invasive measurement of significant substances within biological organisms. The importance of such measurement capability arises not only from the need to observe biochemical reactions in such organisms without disturbance to the system but also in order to help control chronic diseases such as diabetes, where it is highly desirable to measure the patients blood glucose levels much more frequently than is practical, when puncturing the skin is required. Molecular spectroscopy has been proposed to make such measurements. However, the blood and interstitial fluids contain a very great number of compounds which must be distinguished. Absorption spectroscopy in the visible or near infrared suffers from the difficulty that the spectrum of many compounds that are present in the blood and other tissues substantially overlap in this region. Mid-IR spectroscopy produces spectra which are considerably more unique to individual molecules but suffers from two serious problems: (1) Detectors must be operated at cold temperatures if they are to be sufficiently sensitive, and (2) Water absorbs mid-IR radiation strongly and such radiation can only penetrate a few tens of microns into an organism.
Raman spectroscopy has been proposed to obviate some of these difficulties. In Raman spectroscopy, a scattering spectrum is produced at frequencies which are at the difference or sums of the frequencies of the illuminating radiation and the characteristic spectral frequencies of the molecule. Difference frequency generation is referred to as Stokes scattering, and sum frequency generation is referred to as Anti-Stokes scattering. The resulting spectral signatures are advantageously particular to the analytes of interest. However, the cross-sections for Raman scattering are small, and the resulting scattered signals are weak. Weak signals can also arise from spectroscopies that use other non-linear processes or where the available power from the light source is small. Other representative examples would include four wave mixing, frequency doubling, and multi-photon fluorescence.
U.S. Pat. No. 6,064,897, entitled “Sensor utilizing Raman spectroscopy for non-invasive monitoring of analytes in biological fluid and method of use,” proposes the use of a multiplicity of bandpass filters and detectors to monitor a multiplicity of significant spectral lines emerging from the analyte of interest. The premise of the method is that a multiplicity of spectral lines is better correlated to any particular analyte than a single line, in the presence of other substances that may have confounding spectra. In addition, the patent presents a system using discrete transmission filters, which can have small attenuation. Such systems, however, may be limited in sensitivity by detector noise. The dark current of detectors scales adversely with increasing detector area. A multiplicity of detectors will therefore, in aggregate have approximately Nd times the total dark current of an individual detector, where Nd is the number of detectors. Because the dark current can be algebraically subtracted from the signal, the noise contribution arises from its variance, rather than from the mean value. The variance will be proportional to (Nd)1/2. The approach, described in U.S. Pat. No. 6,064,897, therefore suffers from the difficulty that the aggregate noise scales with the number of detectors.
Raman scattering has also been proposed in the aqueous humor of the eye to measure glucose concentrations, as in U.S. Pat. No. 6,181,957. The aqueous humor has desirable optical properties such as high transparency. However, it is highly desirable to perform such monitoring through the skin so as to be able to continuously measure the relevant analytes. Also, serious issues of eye safety are entailed with the proposed method. Irrespective of the choice of measurement location, U.S. Pat. No. 6,181,957 also does not propose a method to resolve the problem of measuring weak scattered signals with practical detectors.
Raman scattering to measure multiple analytes in blood was reported in thesis work by T. W. Koo in a dissertation entitled, “Measurement of blood analytes in turbid biological tissue using near infrared Raman spectroscopy,” published by MIT, in August 2001. Weak Raman signals are reported with as little as 6 counts per every 10 seconds for glucose. Long measurement times and high laser power is required (300 seconds, and 280 mW). These parameters are not practical for many applications.
In other work, glucose measurements were made, in vivo, using Raman scattering where light was introduced through the finger tip {“Noninvasive blood analysis by tissue modulated NIR Raman spectroscopy,” J. Chaiken et al., in Proceedings of SPIE Vol. 4368, p. 134 (2001)}. The method improves the signal size but still uses cooled detectors, high laser power, and a low f number spectrometer that is expensive. The basic problem of weak signals remains unresolved.
Another difficulty, which has been of great importance in noninvasive measurements is the establishment of a reliable calibration for a wide variety of patients, that will remain valid over varying conditions and over time. Variations arise from many sources including the following: (1) Temperature, (2) Presence of varying concentrations of confounding substances with overlapping spectra, (3) Presence of other substances which affect the spectrum of the analyte either in regard to the amplitude, shape, or position of the spectral lines, (4) Variations in the location of the sampling, and in particular the fraction of blood, and interstitial fluid that may be therein, and (5) Drifts in the instrument including the wavelength of sources or of spectroscopic optical components.
Calibration has been sought through regression techniques, based on the spectra of multiple substances, obtained by measuring the individual amplitudes of many spectral lines. Such techniques remain sensitive to variations in the size and constituency of the sample volumes, and also result in much more complex spectrometers. The work of Chaiken et al. adds thereto a method based on subtracting signals using spectra obtained from a finger without pressure, with respect to a pressed finger. Referring to FIG. 11 of the aforementioned reference, there is still much scatter in the correlation between the Raman measurement and laboratory measurements of glucose, rendering the technique disadvantageously inaccurate.